Pease help diagnosing my 2235


lagunablue94c
 

Hello everyone,
First off I'd like to thank you for having a group like this who might help a poor chump like me. 
I'm an electronics novice trying to learn more so I bought a 2235 for $70 with the understanding that one channel works.  Well, upon initial startup only one trace showed up.  After making some quick adjustments I was delighted to find traces for both channels.  Unfortunately after further investigation it seems neither channel works correctly.
I've searched through the Textronics troubleshooting manual, service manual, operators manual and schematic trying to understand my problems but I'm not knowledgeable enough to make a diagnosis.  I'm hoping someone here could give me a little insight so here's what's wrong:
  1. Intensity knob only adjusts from bright to really bright
  2. Vertical line on left hand side of screen
  3. Line running through waveform - looks like the electron gun moving to start position
  4. Waveform is noisy
  5. When measuring a low voltage signal, the signal is added to a phantom 24v sine wave
  6. Delay trace shows up as a vertical line on the left side of the screen
Here of some pictures of this scope in action:
From tektronix">pic1
Both channels measuring 500mA internal reference signal.  Seems ok.

From tektronix">pic2
120vac signal. Note vertical line on left side & lighter line running through middle of trace.

From tektronix">pic3
Other end of 120vac signal.  Note light horizontal line coming from end of trace.

From tektronix">pic4
Upper trace is the line-in to an audio amplifier.  Lower trace is the amplified output.  It's difficult to see it here, but the real signal is added to a 24v sine wave.

From tektronix">pic5
Lower trace is measuring the 500mA internal reference signal.  Vertical line in upper left corner is the delay trace.

I can't see anything obviously wrong under the cover.  There is some discoloration of the mainboard around some of the big power resistors but that's probably to be expected.

If anyone has any ideas I'd really appreciate it.

Thanks,
Bryan


Lindberg, Steve <steve.lindberg@...>
 

The line going back is retrace.

Check power supplies, especially around focus and intensity stuff in HV section. Make sure you don't get shocked.

 

Steve Lindberg

 

From: TekScopes@... [mailto:TekScopes@...] On Behalf Of lagunablue94c
Sent: Wednesday, May 18, 2011 12:59 PM
To: TekScopes@...
Subject: [TekScopes] Pease help diagnosing my 2235

 

 

Hello everyone,
First off I'd like to thank you for having a group like this who might help a poor chump like me. 
I'm an electronics novice trying to learn more so I bought a 2235 for $70 with the understanding that one channel works.  Well, upon initial startup only one trace showed up.  After making some quick adjustments I was delighted to find traces for both channels.  Unfortunately after further investigation it seems neither channel works correctly.
I've searched through the Textronics troubleshooting manual, service manual, operators manual and schematic trying to understand my problems but I'm not knowledgeable enough to make a diagnosis.  I'm hoping someone here could give me a little insight so here's what's wrong:

1.      Intensity knob only adjusts from bright to really bright

2.      Vertical line on left hand side of screen

3.      Line running through waveform - looks like the electron gun moving to start position

4.      Waveform is noisy

5.      When measuring a low voltage signal, the signal is added to a phantom 24v sine wave

6.      Delay trace shows up as a vertical line on the left side of the screen

Here of some pictures of this scope in action:
From tektronix">pic1
Both channels measuring 500mA internal reference signal.  Seems ok.

From tektronix">pic2
120vac signal. Note vertical line on left side & lighter line running through middle of trace.

From tektronix">pic3
Other end of 120vac signal.  Note light horizontal line coming from end of trace.

From tektronix">pic4
Upper trace is the line-in to an audio amplifier.  Lower trace is the amplified output.  It's difficult to see it here, but the real signal is added to a 24v sine wave.

From tektronix">pic5
Lower trace is measuring the 500mA internal reference signal.  Vertical line in upper left corner is the delay trace.

I can't see anything obviously wrong under the cover.  There is some discoloration of the mainboard around some of the big power resistors but that's probably to be expected.

If anyone has any ideas I'd really appreciate it.

Thanks,
Bryan


Tom Miller <tmiller@...>
 

Ok, here is what you need to do first.
 
Go to section five of the service manual, "Adjustment Procedures" and start doing steps 1 to 6. Look at the AC ripple on each of the low voltage DC test points. Record each of the levels. Be vary careful doing this as a slip of a probe can make things much worse, On page 6-8, the voltages and ripple limits are given.
 
Tell us what the results are. You will need a high voltage probe for your DC meter to measure the cathode voltage correctly. It will be about 2000 volts negative.
 
At first glance, it looks like the grid bias is up too high.
 
Regards,
Tom
 
 

----- Original Message -----
Sent: Wednesday, May 18, 2011 3:59 PM
Subject: [TekScopes] Pease help diagnosing my 2235

 

Hello everyone,
First off I'd like to thank you for having a group like this who might help a poor chump like me. 
I'm an electronics novice trying to learn more so I bought a 2235 for $70 with the understanding that one channel works.  Well, upon initial startup only one trace showed up.  After making some quick adjustments I was delighted to find traces for both channels.  Unfortunately after further investigation it seems neither channel works correctly.
I've searched through the Textronics troubleshooting manual, service manual, operators manual and schematic trying to understand my problems but I'm not knowledgeable enough to make a diagnosis.  I'm hoping someone here could give me a little insight so here's what's wrong:

  1. Intensity knob only adjusts from bright to really bright
  2. Vertical line on left hand side of screen
  3. Line running through waveform - looks like the electron gun moving to start positio n
  4. Waveform is noisy
  5. When measuring a low voltage signal, the signal is added to a phantom 24v sine wave
  6. Delay trace shows up as a vertical line on the left side of the screen
Here of some pictures of this scope in action:
From tektronix">pic1
Both channels measuring 500mA internal reference signal.  Seems ok.

From tektronix">pic2
120vac signal. Note vertical line on left side & lighter line running through middle of trace.

From tektronix">pic3
Other end of 120vac signal.  Note light horizontal line coming from end of trace.

From tektronix">pic4
Upper trace is the line-in to an audio amplifier.  Lower trace is the amplified output.  It's difficult to see it here, but the real signal is added to a 24v sine wave.

From tektronix">pic5
Lower trace is measuring the 500mA internal reference signal.  Vertical line in upper left corner is the delay trace.

I can't see anything obviously wrong under the cover.  There is some discoloration of the mainboard around some of the big power resistors but that's probably to be expected.

If anyone has any ideas I'd really appreciate it.

Thanks,
Bryan


magnustoelle
 

G'd Day Bryan,

and welcome to the group.

Just to amend to Tom Miller's excellent advice: Do not compromise on any safety aspects when doing high voltage probing.

You might want to check the usual online-auctions for a SIMPSON High Voltage probe or similar. I am using a SIMPSON 00509 probe for some time now and I am happy with it.

Good Luck with your troubleshooting.

Magnus


Kevin Wood G7BCS
 

Hi Bryan,

In addition to the excellent suggestions already made, if the cathode is
OK at about -2KV I would have a careful look at the DC restorer circuit
that drives the control grid of the tube.

I.E. C855, CR851, 853, 854, 855 and associated components

This circuit takes the Z axis signal from the intensity control
incorporating the blanking pulse when the timebase retraces (a few tens of
volts at a guess) and references it to the cathode voltage (-2KV if all is
well) so it can control the grid of the tube relative to the cathode.

I had a scope with exactly the same symptoms of excessive brightness and a
couple of the diodes in the restorer circuit were to blame.

While you're in the area, check the values of all the resistors. The 510k
resistors in the focus chain are particularly prone to going high in
value, although this doesn't appear to be your main problem there may be
other resistors going faulty.

As for the AC signal that's superimposed on the trace, can you estimate
its' frequency? I agree that this is probably down to ripple on one of the
power supply rails. Checking its' frequency relative to the two supply
choppers might give you some clues - in addition to just checking the
rails for ripple, of course.

Kevin


Greg
 

"lagunablue94c" <bryanlow70@...> wrote:

1. Intensity knob only adjusts from bright to really bright
2. Vertical line on left hand side of screen
3. Line running through waveform - looks like the electron gun
moving to start position
4. Waveform is noisy
5. When measuring a low voltage signal, the signal is added to a
phantom 24v sine wave
6. Delay trace shows up as a vertical line on the left side of
the screen

Thanks,
Bryan
In part make sure you understand all the controls.
There are two brightness controls. The delay controls
are some what confusing (to me) and take some prctice,
but I am not saying your don't have an issue. What
I am saying when I first got mine I though something
was wrong, but it was OK and I just did not understand
the operation of the 2235a.

Bryan I have a 2235a and love it. I had a few issues.
I can't address your specifics, but all my problems
were with the two vertical attenuator's X1 and X10 (per
channel). They are very delicate and subject go failure.
Some times you can unplug them, pop off the snap-on
plastic cover and re-solder some connection.

Obviously your issues sound more in-depth, but I thought
I share that with you.


lagunablue94c
 

Thank you all for the suggestions.
I think I'm a little in over my head here, nevertheless it is a fascinating exercise and a great learning experience.
Based on what you guys have said, I'm making some assumptions:
  1. There is a ripple in the CRT PS.  This would explain the superimposed 60Hz AC signal when measuring an audio signal.  What I don't understand is why the same thing is not happening when the internal reference signal is measured.
  2. There are some resistors or diodes out of spec in the Z-axis circuitry
  3. The DC Restorer circuitry is not working properly.  Kevin, I'm trying to find reference to the blanking pulse during retrace--Is this excerpt from pg. 3-17 it?
  • "Fast-rise and fast-fall transitions of the Z-axis output signal are coupled to the crt control grid through C855 to start the crt writing-beam current toward the new intensity level.  The Dc Restorer output level then follows the Z-axis output voltage level to set new bias voltage for the crt control grid."
Reading this stuff is really making my head smoke!
So here's my dilemma: The manual lists a bunch of equipment needed to make adjustments, none of which I can really afford.  My entire test-bench consists of a Fluke 77 DMM and a broken Tektronix 2235.

Based on the above assumptions I've ordered a Fluke 80k-6 probe to check the CRT voltages and now I'm planning to get an ESR meter to check the caps in and around the PS. 

Is there any other piece of equipment I absolutely need to have to make this diagnosis?

Thanks again,
Bryan


Kevin Wood G7BCS
 

3. The DC Restorer circuitry is not working properly. Kevin, I'm
trying to find reference to the blanking pulse during retrace--Is this
excerpt from pg. 3-17 it?

* "Fast-rise and fast-fall transitions of the Z-axis output signal
are coupled to the crt control grid through C855 to start the crt
writing-beam current toward the new intensity level. The Dc Restorer
output level then follows the Z-axis output voltage level to set new
bias voltage for the crt control grid."
Reading this stuff is really making my head smoke!
Yep, that's it. You can regard it as a network that turns the Z axis
signal (let's say it varies from 0-100V for the sake of argument. I'm not
sure on the exact range) into an AC signal with an amplitude of 0-100v.
That signal is then coupled into the cathode circuit where it's rectified
again, so it appears as a -2000V to -2100V signal. C855 bypasses it to
improve the transient response.

So here's my dilemma: The manual lists a bunch of equipment needed to
make adjustments, none of which I can really afford. My entire
test-bench consists of a Fluke 77 DMM and a broken Tektronix 2235.
Based on the above assumptions I've ordered a Fluke 80k-6 probe to check
the CRT voltages and now I'm planning to get an ESR meter to check the
caps in and around the PS.

Is there any other piece of equipment I absolutely need to have to make
this diagnosis?
I wouldn't say so at this stage. The DC restorer just needs the diodes and
resistor values around it checked. That can be done with the fluke.

Only thing that might be handy is another scope to check for ripple on the
power supply circuitry although if the ripple is not present when
measuring the calibrator signal it makes me wonder if it's the ground
connection in your probe that's the problem.

Kevin


lagunablue94c
 

It really is strange that I don't have any ripple visible in the
calibrator signal. The scope came with three good probes, and all of
them react similarly on both channels so I don't think the problem is
the probes themselves. Could the probes possibly be referenced to
something other than ground, and there's a ripple in the reference
voltage?

Only thing that might be handy is another scope to check for ripple on
the
power supply circuitry although if the ripple is not present when
measuring the calibrator signal it makes me wonder if it's the ground
connection in your probe that's the problem.

Kevin


Tom Miller <tmiller@...>
 

For #1, where in the CRT power supply did you measure the ripple? Do you have a bad probe ground? Are you trying to measure the audio in a floating system?
 
2. Out of spec resistors are known causes of display problems in the 2235 (and other scopes). There are some carbon comp resistors R888-R893, 510K that are known to drift high. Just lift one lead and measure them. Replace the ones that are out of  spec.
 
3. The HV probe is a very useful thing to have and will save your 77 from an oops.
 
4. You will only need the expensive equipment to do a full calibration for horizontal sweep and vertical levels.
 
 
Tom
 
 

----- Original Message -----
Sent: Friday, May 20, 2011 12:34 PM
Subject: [TekScopes] Re: Pease help diagnosing my 2235

 

Thank you all for the suggestions.
I think I'm a little in over my head here, nevertheless it is a fascinating exercise and a great learning experience.
Based on what you guys have said, I'm making some assumptions:

  1. There is a ripple in the CRT PS.  This would explain the superimposed 60Hz AC signal when measuring an audio signal.  What I don't understand is why the same thing is not happening when the internal reference signal is measured.
  2. There are some resistors or diodes out of spec in the Z-axis circuitry
  3. The DC Restorer circuitry is not working properly.  Kevin, I'm trying to find reference to the blanking pulse during retrace--Is this excerpt from pg. 3-17 it?
  • "Fast-rise and fast-fall transitions of the Z-axis output signal are coupled to the crt control grid through C855 to start the crt writing-beam current toward the new intensity level.  The Dc Restorer output level then follow s the Z-axis output voltage level to set new bias voltage for the crt control grid."
Reading this stuff is really making my head smoke!
So here's my dilemma: The manual lists a bunch of equipment needed to make adjustments, none of which I can really afford.  My entire test-bench consists of a Fluke 77 DMM and a broken Tektronix 2235.

Based on the above assumptions I've ordered a Fluke 80k-6 probe to check the CRT voltages and now I'm planning to get an ESR meter to check the caps in and around the PS. 

Is there any other piece of equipment I absolutely need to have to make this diagnosis?

Thanks again,
Bryan


Tom Miller <tmiller@...>
 

If I remember right, is there a separate ground wire on the input BNC connector(s) on these scopes?
 
Tom
 

----- Original Message -----
Sent: Friday, May 20, 2011 1:23 PM
Subject: [TekScopes] Re: Pease help diagnosing my 2235

 

It really is strange that I don't have any ripple visible in the
calibrator signal. The scope came with three good probes, and all of
them react similarly on both channels so I don't think the problem is
the probes themselves. Could the probes possibly be referenced to
something other than ground, and there's a ripple in the reference
voltage?

> Only thing that might be handy is another scope to check for ripple on
the
> power supply circuitry although if the ripple is not present when
> measuring the calibrator signal it makes me wonder if it's the ground
> connection in your probe that's the problem.
>
> Kevin
>


lagunablue94c
 


--- In TekScopes@..., "Tom Miller" wrote:
>
> For #1, where in the CRT power supply did you measure the ripple? Do you have a bad probe ground? Are you trying to measure the audio in a floating system?
>
I haven't measured ripple yet.  I'd need a scope for that :( 

The scope came four probes, three of which I believe to be good.  They all give the same readings so I don't think it's a probe problem


Floating system?  Hmm, yes I believe it is.  Could this be causing the superimposed signals problem?  The amplifier in question is a simple 8w mono amp.  Power input is +12v -12vdc.  No ground. 

Here are a few more pictures for reference:
From tektronix">pic1 
probe not touching

From tektronix">pic2 probe touching  (+)speaker out,  Amp NOT PLUGGED IN.  NO VOLTAGE PRESENT.

From tektronix">pic3
probing (+)speaker out.  AMP PLUGGED IN, NO AUDIO PLAYING.  Why would these waveforms be showing up, and if this was really a PS ripple wouldn't the waveform be smooth, and without these "kinks" in it?

From tektronix">pic4
Probing (+)speaker out,  PLAYING MUSIC.

Connecting ground clip to (-)speaker out has no apparent effect on waveforms.


> 2. Out of spec resistors are known causes of display problems in the 2235 (and other scopes). There are some carbon comp resistors R888-R893, 510K that are known to drift high. Just lift one lead and measure them. Replace the ones that are out of spec.
>

Is there any way to check a resistor without breaking out the soldering iron?  I was thinking about measuring voltage drops, but that would require making an assumption about current flowing.


Thanks,
Bryan


Kevin Wood G7BCS
 

Here are a few more pictures for reference:
I would fix the intensity problem first if I were you.Running the tube as
bright as it looks in those pictures for any length of time will do it no
favours at all.

Check over all the components in the cathode & grid drive / DC restorer
and see if you can crack that. Then you can focus on the hum without
worrying about burning the tube.

Kevin


lagunablue94c
 

Okay, were getting closer.  Chalk that up to user error I guess.

From tektronix">Pic1 
Connected Amp ground to the banana plug jack on the front panel labeled "GND" duh...

From tektronix">Pic2
Whatever that clip is, it's not connected to chassis ground.


--- In TekScopes@..., "Tom Miller" wrote:
>
> If I remember right, is there a separate ground wire on the input BNC connector(s) on these scopes?
>
> Tom
>


Robert Simpson
 

Bryan,
If you are working on non grounded devices, you must learn about differential probe techniques.
join
http://www.ebaman.com/index.php/home
then search on differential, Tek has a great article. Otherwise you will get all kinds of screwy results.
Bob

--- On Fri, 5/20/11, lagunablue94c <bryanlow70@...> wrote:


From: lagunablue94c <bryanlow70@...>
Subject: [TekScopes] Re: Pease help diagnosing my 2235
To: TekScopes@...
Date: Friday, May 20, 2011, 12:56 PM


 




---


Tom Miller <tmiller@...>
 

Well, you are making some forward progress. A very fast check of a resistor is to measure the resistor in circuit. If you get a reading larger than the value marked on the resistor, you know it is bad.
 
There is no lazy way to repair this stuff and learn anything, so go ahead and get the iron out. Make sure you get an ESD safe iron that has the tip grounded. Failure to use an ESD safe iron is sudden death to expensive and hard to find parts.
 
You could make measurements of voltages in a working circuit but for these, you will need a good HV probe for the DMM. Also, you will need to make many calculations to find out what you are seeing.
 
Someone (Kevin Wood) has made an important comment about fixing the intensity problem first. You can burn in the screen with the brightness up so high.
 
Take your ohmmeter and measure the resistance from the input BNC connector outer shell to the ground point of the scope. Report your findings.
 
 
Good luck,
Tom
 
 
 
 

----- Original Message -----
Sent: Friday, May 20, 2011 3:56 PM
Subject: [TekScopes] Re: Pease help diagnosing my 2235

 


--- In TekScopes@..., "Tom Miller" wrote:
>
> For #1, where in the CRT power supply did you measure the ripple? Do you have a bad probe ground? Are you trying to measure the audio in a floating system?
>
I haven't measured ripple yet.  I'd need a scope for that :( 

The scope came four probes, three of which I believe to be good.  They all give the same readings so I don't think it's a probe problem


Floating system?  Hmm, yes I believe it is.  Could this be causing the superimposed signals problem?  The amplifier in question is a simple 8w mono amp.  Power input is +12v -12vdc.  No ground. 

Here are a few more pictures for reference:
From tektronix">pic1 
probe not touching

From tektronix">pic2 probe touching  (+)speaker out,  Amp NOT PLUGGED IN.  NO VOLTAGE PRESENT.

From tektronix">pic3
probing (+)speaker out.  AMP PLUGGED IN, NO AUDIO PLAYING.  Why would these waveforms be showing up, and if this was really a PS ripple wouldn't the waveform be smooth, and without these "kinks" in it?

From tektronix">pic4
Probing (+)speaker out,  PLAYING MUSIC.

Connecting ground clip to (-)speaker out has no apparent effect on waveforms.


> 2. Out of spec resistors are known causes of display problems in the 2235 (and other scopes). There are some carbon comp resistors R888-R893, 510K that are kno wn to drift high. Just lift one lead and measure them. Replace the ones that are out of spec.
>

Is there any way to check a resistor without breaking out the soldering iron?  I was thinking about measuring voltage drops, but that would require making an assumption about current flowing.


Thanks,
Bryan


lagunablue94c
 


So I know this probably is not advisable, but I did it anyway.  I measured the PS ripple in the scope--with the scope.  It seemed to work fine but it looks like there's way too much ripple across all rails. 

150mV p-p at ~21kHz in the 100v rail:
From tektronix">pic1

50mV p-p at ~40kHz on the 30v rail:
From tektronix">pic2

+8.6v, +5.2v and -8.6v rails looked like this:
From tektronix">pic3 
20mV p-p at ~10MHz.

All way out of spec.  I see there's a long thread going on about the same problem here: 2235 with 360 kHz 100mV P-P ringing on +100V PSU rail, so I'll have to read that when I get a chance.

For the most part the scope is working fine but I can still see a little of the retrace and the line on the left hand side is still there:
From tektronix">pic4
Also you can see the waveform gets a little noisy just past the peaks.

Overall though, the scope is very usable, and a helluva deal at $70.
From tektronix">pic5

I'll keep tinkering on this thing until I get it in tip top shape.

Thank you all for the help, and let me know if you have any more advice.

Bryan


Tom Miller <tmiller@...>
 

Just for the hell of it, take a look at the CRT shield to the case ground near the rear of the scope.
 
See the photos in my last message (go to the attachments section).
 
Tom M
 
 
 

----- Original Message -----
Sent: Thursday, May 26, 2011 5:07 PM
Subject: [TekScopes] Re: Pease help diagnosing my 2235

 


So I know this probably is not advisable, but I did it anyway.  I measured the PS ripple in the scope--with the scope.  It seemed to work fine but it looks like there's way too much ripple across all rails. 

150mV p-p at ~21kHz in the 100v rail:
From tektronix">pic1

50mV p-p at ~40kHz on the 30v rail:
From tektronix">pic2

+8.6v, +5.2v and -8.6v rails looked like this:
From tektronix">pic3 
20mV p-p at ~10MHz.

All way out of spec.  I see there's a long thread going on about the same problem here: 2235 with 360 kHz 100mV P-P ringing on +100V PSU rail, so I'll have to read that when I get a chance.

For the most part the scope is working fine but I can still see a little of the retrace and the line on the left hand side is still there:
From tektronix">pic4
Also you can see the waveform gets a little noisy just past the peaks.

Overall though, the scope is very usable, and a helluva deal at $70.
From tektronix">pic5

I'll keep tinkering on this thing until I get it in tip top shape.

Thank you all for the help, and let me know if you have any more advice.

Bryan